how do klingons occupy an uninhabited system?? colony ship will terraform but wont colonize... troop transport won't do it either...also, how do you conduct an attack an occupied system>> my empire is tech level 3, is that too early??

Don't forget to finish terraforming ALL the planets in the system. Build a second colony ship to do so.
It will speed the growth and increase the maximum population. More population means more credits and either intel, science, or production. More credits [and production] means more ships.

Not sure what to tell you, I just tested with ECM playing Klingons and had no issues colonizing any system. Now I did notice the "colonize" button is very dark compared to other buttons, but when hovered it would highlight.